In ink jet printing technology, tiny drops of ink jet printing ink are projected directly on the surface of a substrate for printing without physical contact between the printing device and the substrate surface. The placement of each drop on the printing surface is controlled electronically. The ink jet printing technology has become an important technology for printing variable data and images on paper, cardboard, etc. on the one hand and on products such as, for example, cans, bottles, foils, etc. on the other hand. It is also possible to directly print on fragile objects such as eggs using ink jet technology. The ink jet technology allows to print these data and images at a high speed.
Various principles have been developed for drop generation in the past such as electrostatic, magnetic, piezoelectric, electro-thermal, mechanical micro-valve, and spark discharge technologies. Regardless of type of drop generation, the ink jet technology is basically classified into two basic categories, namely continuous ink jet—CIJ—and impulse or drop-on-demand—DOD—ink jet technology.
The continuous ink jet technology is characterised by pressurised ink discharge through a nozzle to generate drops of ink directed to the substrate surface in a continuous stream.
The impulse ink jet technology differs from the continuous ink jet technology in that the ink supply is maintained at or near atmospheric pressure. An ink drop is ejected from a nozzle only on demand when a controlled excitation is applied to the drop-generating transducer. The impulse ink jet (or drop-on-demand) technology is primarily used in colour printers.
In recent years colour printers have been developed allowing to produce colour prints with the basic colours yellow, cyan and magenta and occasionally black. These colours are widely used as process colours in established printing processes like lithography, gravure and flexo printing.
In order to further improve the quality of the colour prints, especially in view of the increasing applications for photo printing, so called “spot colours” are used. In offset printing a spot colour is any colour generated by a pure or mixed ink that is printed using a single run. These spot colours can include orange and green in addition to the four basic colours or any other colours which expand the colour gamut and vividness of a printed image.
EP 1 205 521 discloses a pigment preparation for an ink jet process according to which a wide variety of pigments can be used. When producing the pigment preparation all components are comminuted in a mill to obtain a particle size wherein preferably 99% of the particles have a particle size of below 15 μm. Pigments having a particle size of below 1 μm are more or less of spherical nature which are not able to produce a metallic appearance in a print.
JP 11-323223 A discloses an ink jet printing ink containing metallic pigments produced with pvd methods.
Also, WO 2006/101054 A1 describes ink jet printing inks containing pvd-metallic pigments.
However, it turned out that these pvd metallic pigments of the prior art do settle and aggregate when being comminuted. When introducing these comminuted pvd-pigments in printing inks, the aggregates can not be easily disaggregated and are likely to clog the ink jet nozzles of the printer. Moreover, the aggregated pvd metallic pigments have a diminished metallic luster compared to non-aggregated pvd metallic pigments.